Defecation:The
process of defecation is controlled in the anus by
reflexes called defecation
reflexes. It works as follows.
Feces which
enters the rectum distends it
and also distends its
myenteric
plexuses
deep in its
muscularis externa.
The plexuses excites
the
formation of peristaltic
wave which develops retrograde and prograde in
the descending colon, sigmoid
colon and rectum. The effect
of these peristaltic
waves is to force feces
from these locations
towards the anus.
As this peristaltic wave reaches
the anus it automatically releases
the internal anal sphincter while the external anal sphincter is consciously relaxed; feces comes out in the
process
known as defecation.